This Sunday, we celebrate the Feast of the Holy Trinity which is a feast of a
doctrine rather than of an event of salvation. Historically, this started in the year 1030 on
the First Sunday after Pentecost. Pope John XXII approved it in 1334 as a Feast of the
Universal Church.
The name trinity means “three in one” - three in one because the Three Divine
Persons: Father, Son and Holy Spirit are not three Gods but one God. Maybe hard to understand but it is the mystery of God in Him. St. Augustine said, “Trinity is not a problem to be solved but a mystery to be lived”.
On the doctrinal level, the Catechism of the Catholic Church (nos. 253-255) summarizes
this doctrine into three: that the Trinity is One. We do not confess Three Gods but One God in Three Divine Persons, the Consubstantial Trinity. The Divine Persons do not share divinity among the Three but each of the Three divine Persons is God, whole and entire: the Father is that which the Son is, that which the Father is, the Father and the Son that which the Holy Spirit is, that is, by nature One God.
Second, the Divine Persons are really distinct from one another. God is one but not solitary. The Father, the Son and the Holy Spirit are not simply names designating modalities of the divine being, for they are really distinct from one another. He is not the Father who is the Son nor is the Son He who is the Father, nor is the Holy Spirit who is the Father or the Son. They are distinct from one another in their relations of origins: it is the Father who generates, the Son who is begotten and the Holy Spirit who proceeds.
Third, the Three Divine Persons are relative to one another. It is because the Three does not divide the divine unity, the real distinction of the Persons from one another resides solely in relationships which relate the Three to one another. In the relational names of the Persons, the Father is related to the Son, the Son to the Father and the Holy Spirit
to both. While the Three are called Three Persons in view of their relations, we believe in one nature or substance…because of the unity, the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son.